This set practises five skills: tracing carbon, linking decomposition to recycling, reading growth phases, naming limiting factors, and telling density dependent effects from one-off events. Questions run from easy to harder. All data are invented for practice.
Attempt each one before opening the answer. The overview of the topic is nutrient cycles and populations.
Questions
1. (Easy) Name the process that moves carbon from carbon dioxide in the air into a leaf.
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Photosynthesis. Carbon dioxide is used to make glucose, so the carbon is now in the organic compounds of the plant.
2. (Easy) A fox eats a rabbit. Name the process that passes carbon from the rabbit to the fox.
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Feeding. Organic compounds in the rabbit’s tissues are digested and absorbed by the fox.
3. (Easy) Give two ways carbon dioxide returns to the air from living or dead organisms.
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Respiration (by plants, animals, fungi and bacteria, including decomposers) and combustion of wood or fossil fuels.
4. In a grassland, photosynthesis takes in 200 units of carbon in a year. Plant respiration releases 90, animal respiration releases 10 and decomposer respiration releases 85. Calculate the net change in carbon stored by the grassland.
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Released = 90 + 10 + 85 = 185. Net change = 200 − 185 = +15.
The grassland stores 15 more units than it releases, so it is gaining carbon that year.
5. Two bags each start with 50 g of dry leaves. After six weeks the bag kept at 10 °C has 40 g left, and the bag kept at 25 °C has 22 g left. Calculate the percentage mass lost in each, and explain the difference.
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10 °C: lost 50 − 40 = 10 g, so 10/50 × 100 = 20%.
25 °C: lost 50 − 22 = 28 g, so 28/50 × 100 = 56%.
At 25 °C decomposer enzymes and respiration work faster, so more of the leaf is digested. The mass lost leaves mainly as carbon dioxide from respiration.
6. Explain why dead leaves last longer in waterlogged soil than in well-drained soil.
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Water fills the air spaces, so less oxygen reaches the soil. Many decomposers respire aerobically, so their respiration and activity slow down. Less material is digested, so the leaves break down more slowly.
7. A culture of bacteria gives these counts (invented): 0 h, 500; 1 h, 520; 2 h, 1000; 3 h, 2000; 4 h, 4000; 5 h, 7000; 6 h, 8000; 7 h, 8000. (a) Name the phase from 0 to 1 h. (b) Show that growth from 2 to 4 h is exponential. (c) Calculate the percentage increase from 4 to 5 h and say what it shows.
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(a) Lag phase. The count hardly changes (500 to 520).
(b) 1000 → 2000 → 4000. The count doubles each hour, the same factor each time, so growth is exponential.
(c) Increase = 7000 − 4000 = 3000. 3000/4000 × 100 = 75%. This is less than the 100% of a doubling, so growth is slowing.
8. For the bacteria in Question 7, explain the plateau from 6 to 7 h.
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The count stays at 8000, so births roughly equal deaths. A resource such as nutrients or oxygen has become limiting, or waste has built up, so the population stops growing. This is the stationary phase.
9. Duckweed is grown with different nitrate levels (invented): 0 mg per litre, 10 fronds; 5, 30; 10, 50; 15, 55; 20, 55. (a) Calculate the increase in fronds per 1 mg per litre between 0 and 10. (b) At 20 mg per litre, is nitrate limiting? Explain.
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(a) 50 − 10 = 40 fronds over 10 mg per litre, so 4 fronds per 1 mg per litre.
(b) No. From 15 to 20 the count stays at 55, so extra nitrate does not help. Another factor, such as light or space, now limits growth.
10. (Harder) A population of 1200 voles falls to 300 after a flood. In another year, with no flood, the population rose to 2000, and then 40% died of a disease that spreads by contact. (a) Calculate the percentage fall after the flood. (b) Calculate the number dying of disease. (c) Decide which event is density dependent and justify it.
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(a) Fall = 1200 − 300 = 900. 900/1200 × 100 = 75%.
(b) 40% of 2000 = 0.4 × 2000 = 800 voles.
(c) The disease is density dependent, because contact spreads it more easily when the population is crowded at 2000. The flood is a one-off event: it acts whatever the crowding.
If you got these wrong
- Mixed up processes or left out a process word (Questions 1 to 4): revisit tracing carbon through a supplied system.
- Decomposition, temperature or oxygen (Questions 5 and 6): see linking decomposition with nutrient recycling.
- Phases, doubling or percentage increase (Questions 7 and 8): see interpreting population growth phases.
- Plateau or limiting factor (Question 9): see explaining a limiting factor from data.
- Cause of a crash or density dependence (Question 10): see distinguishing density dependence from a one-off event.
Record each slip in the mistake log and retest queue so that you revise the lesson you actually need.
If the same slip keeps returning, online one-to-one Biology tuition lets a teacher work on that pattern with new questions.